A control device for an alternating current motor is conventionally known, which has a memory for storing beforehand, in the form of table, current command calculation data for individual rotational angular regions of the motor, and which calculates current commands of individual phases on the basis of the data read out from the memory in accordance with a region to which an actual rotational position of the motor belongs. For example, a vector control device of FIG. 4 for controlling the drive of a three-phase induction motor is known.
In the control device of FIG. 4, the difference between a speed command Vc read out from a program (not shown) by a processor (not shown) for vector control and an actual speed .omega.r detected by a single speed detector PC for detecting the actual rotational speed .omega.r of the three-phase induction motor 6 is amplified by an amplifier 1, to thereby generate a torque command T. Then, a secondary current command I.sub.2 is derived in an element 2 by dividing the torque command T by an exciting magnetic flux command .PHI. supplied from an element 8, and a slip frequency .omega.s is derived in an element 10 by dividing the product of a proportional constant K2 and the secondary current command I.sub.2 by the exciting magnetic flux command .PHI.. Further, the slip frequency .omega.s and the actual speed .omega.r are added together by an adder 11, to thereby derive a primary current phase .theta., and the exciting magnetic flux command .PHI. is divided in an element 9 by a proportional constant K1 to derive an exciting current component I.sub.0. Furthermore, in a current operation circuit 3, a primary current command I.sub.1 is determined on the basis of the exciting current component I.sub.0 and the secondary current command I.sub.2.
A three-phase converter 4 has a memory for storing sinusoidal data of individual phases in association with respective angular regions of the primary current phase .theta. in the form of table, and determines current commands of respective phases, IU (=I.sub.1 .times.sin.theta.), IV (=I.sub.1 .times.sin(.theta.-2.pi./3)) and IW (=I.sub.1 .times.sin.theta.-4.pi./3)), by multiplying the primary current command I.sub.1 by sinusoidal data sin.theta., sin(.theta.-2.pi./3) and sin(.theta.-4.pi./3) of respective phases read out from the memory in accordance with the input primary current phase .theta.. Further, a current controller 5 carries out current control such that the differences between actual currents of respective phases detected by current detectors CTU, CTV and CTW and the respective current commands IU, IV and IW become zero.
Meanwhile, it is also known to use current commands of two phases, which are derived on the basis of sinusoidal data of two phases stored in the memory of the three-phase converter 4, to derive a current command of the remaining one phase.
As described above, in the current control using the sinusoidal data of respective phases determined for the respective angular regions of the primary current phase .theta., the sinusoidal data is varied in a stepwise fashion as the primary current phase .theta. varies. As a result, the current command of each phase is varied stepwise, causing a torque variation. In particular, a significant torque variation is caused during a low speed operation of the motor.